1. Field of the Invention
The invention relates to a solid electrolyte membrane for a fuel cell that is operable in a medium temperature range, a fuel cell using this solid electrolyte membrane, and manufacturing methods therefor.
2. Description of Related Art
In recent years, fuel cells for generating electric power by an electrochemical reaction between hydrogen and oxygen have attracted attention as an energy source. Amongst fuel cells, fuel cells using solid electrolyte membranes include low temperature fuel cells, such as solid polymer membrane fuel cells, and high temperature fuel cells such as solid oxide fuel cells.
The solid polymer membrane fuel cell uses a polymer membrane, for example, Nafion (a registered trademark of E. I. du Pont de Nemours and Company) as an electrolyte membrane sandwiched between electrodes. When the moisture content of such an electrolyte membrane decreases, the ion conductivity thereof decreases, whereas the resistance thereof increases. Therefore, it is necessary to operate the fuel cell at a low temperature at which extreme evaporation of moisture content can be avoided, in order to limit the resistance of the membrane to within a practical range. Under the present circumstances, the solid polymer membrane fuel cell is ordinarily operated in a temperature range having an upper limit of around 150° C.
The solid oxide fuel cell uses a thin membrane made of an inorganic material, for instance, zirconia, as an electrolyte membrane sandwiched between electrodes. However, there is a tendency for the resistance of such an electrolyte membrane to increase, as the temperature lowers. Thus, it is necessary to operate the fuel cell at a relatively high temperature in order to limit the resistance of such a membrane to within a practical range. Although the resistance of the membrane can be reduced by decreasing the thickness of the electrolyte membrane, it is very difficult to form a dense thin membrane on an electrode formed from a porous material. Therefore, it has not been possible to provide a sufficiently thin electrolyte membrane. Under the present circumstances, the solid oxide fuel cell is ordinarily operated at a temperature that is higher than or equal to about 700° C.
Moreover, in the related art, a hydrogen separation membrane having a five-layer structure, in which both sides of a base metal selected from group VB elements (for example, vanadium (V), niobium (Nb), and tantalum (Ta)) is coated using palladium (Pd) with hydrogen permeable intermediate layers interposed on both sides of the base metal, has been proposed as an example of constituting the membrane from a plurality of layers.
None of the fuel cells using solid electrolyte membranes that are disclosed in such related art operate in a medium temperature range of about 150° C. to about 700° C. Generally, there is a tendency that the higher the temperature, the lower the resistance of the electrolyte membrane. Thus, in the case of the solid polymer membrane fuel cell a high operating temperature is desirable. On the other hand, when the operating temperature of a fuel cell is extremely high, a system using such a fuel cell needs to employ structural members and a configuration that takes into consideration heat resistance. In turn, this leads to constraints on design.